The Islamic State group on Tuesday claimed a series of bombings that killed more than 320 people in Sri Lanka, and released a photo and video of the men it said were responsible.

A local Sri Lankan channel on Tuesday aired a CCTV footage which shows one of the alleged suicide bombers entering the St Sebastian Church in Negombo, moments before a blast ripped through the church during the Easter Sunday mass.

The Islamic State group on Tuesday claimed a series of bombings that killed more than 320 people in Sri Lanka, and released a photo and video of the men it said were responsible.

The massive casualty toll would make the Easter Sunday attacks the deadliest overseas operation claimed by IS since the group proclaimed its caliphate in mid-2014.

The video shows the suspect wearing a backpack, walking towards the church with people walking around him. A man and a girl holding hands were seen crossing paths with the alleged suspect who pauses to pat the girl before he continues to walk towards the church.

An eyewitness, Dilip Fernando, who left the St Sebastian's church right before the blast, claimed to have seen the young man carrying a heavy bag, enter the church at the end of the mass. "He touched my granddaughter's head on the way past. It was the bomber," he was quoted by news agency AFP as saying.

The family wondered why he was entering the church with mass nearly over, Fernando said, adding that the man had looked to be around 30 and "very young and innocent", according to his relatives.

"He was not excited or afraid. He was so calm," he told AFP. Shortly after the man entered the church, there was a massive blast.​

In the CCTV footage, the man is seen entering from the front door of the church through the dozens of people seen offering prayers. He is dressed in a light-blue shirt and black pants.

"Those that carried out the attack that targeted members of the US-led coalition and Christians in Sri Lanka the day before yesterday are Islamic State group fighters," IS propaganda agency Amaq said in a statement. In a later statement, the group gave the noms de guerre of seven people it said were behind the "blessed attack" that targeted Christians during their "blasphemous holiday".

Amaq also released a photo of eight men it said were behind the blasts. Seven of them had their faces covered and three of them held knives. The one man who displayed his bearded face also appeared to be carrying an assault rifle.

Amaq later released a video of the eight fighters pledging allegiance to IS supremo Abu Bakr al-Baghdadi in Arabic, a black IS flag hanging in the background.

The authenticity of the image and video could not be independently verified and the reason for the discrepancy in the reported number of attackers was not immediately clear.

Sunday's bombings targeting churches and high-end hotels are among the deadliest such attacks worldwide since the 2001 strikes on the United States.

The Sri Lankan government on Tuesday blamed the little-known National Thowheeth Jama'ath (NTJ) Islamist group for the blasts, saying they were carried out in retaliation for last month's attacks on two mosques in New Zealand.

Police sources told AFP Tuesday that two Muslim brothers -- sons of a wealthy Colombo spice trader -- were among the perpetrators of the attacks.

They blew themselves up as guests queued for breakfast at the Shangri-La and Cinnamon Grand hotels in the capital, the source said.

The pair were key members of the NTJ, which the government has previously blamed for defacing Buddhist statues, according to an investigation officer.

The IS statement on Tuesday said three fighters it named as Abu Obeidah, Abu Baraa and Abu Moukhtar were behind the attacks on the Shangri-La, Cinnamon Grand and Kingsbury hotels.

Three other fighters it named as Abu Hamza, Abu Khalil and Abu Mohammad carried out attacks on churches in the cities of Colombo, Negombo, and Batticaloa, it said.

The seventh fighter, Abu Abdallah, killed three police officers in an attack in a Colombo suburb, it said.

Tuesday's claim comes one month after a Kurdish-led Syrian force announced the fall of IS's self-declared "caliphate", after routing jihadists from their last holdout in east Syria with backing from a US-led coalition.

The jihadists retain a global network of recruits and have claimed attacks in Iraq, Syria and beyond.

On Sunday, IS claimed responsibility for an attack that killed 10 people in the Afghan capital Kabul the previous day.

Ethnic and religious violence has plagued Sri Lanka for decades, with a 37-year conflict with Tamil rebels followed by an upswing in recent years of clashes between the Buddhist majority and Muslims.